A seriatim opinion describes an opinion delivered by a court with multiple judges, in which each judge reads his or her own opinion rather than a single judge writing an opinion on behalf of the entire court. This is a practice generally used when a case does not have a majority opinion.
Most frequently used in modern times (when used at all) pleadings as a shorthand for "one by one in sequence". For example "save as expressed admitted herein, each allegation of the plaintiffs is denied as if set out in full and traversed herein seriatim."
Also sometimes seen in older deeds and contracts as a more traditional way of incorporating terms of reference. For example "the railway by-laws shall apply to the contract as if set out herein seriatim."
Use of the word (and other Latin phrases) has become less and less frequent in legal documentation as a result of a concerted campaign by certain NGO pressure groups for "plain English" legal documentation, more readily understandable by the public.